Methods of producing a package for semiconductor chips

ABSTRACT

The inventive method is based on the a idea of releasing a mechanical connection between the semiconductor chip and the supporting substrate during the manufacturing of the packing. The mechanical connection required for producing the electrical contacts between the semiconductor chip and the supporting substrate ensues only temporarily. As a result, a critical interface in the packing is removed thereby resulting in distinctly reducing the thermomechanical stresses.

CLAIM FOR PRIORITY

This application is a national stage of PCT/EP02/07439, published in theGerman language on Jan. 23, 2003, which claims the benefit of priorityto German Application No. DE 101 33 361.7, filed on Jul. 10, 2001.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method of producing a package forsemiconductor chips, and in particular, to a method of producingminiature packages or miniature housings for semiconductor chips.

BACKGROUND OF THE INVENTION

With the increasing integration in information and communicationtechnology, the requirements imposed on the constructing and connectingtechniques are becoming ever greater. Ever higher requirements areimposed in particular on the package of the semiconductor chips. Thepackage is the main medium for connecting the integrated circuit on thesemiconductor chip to the rest of the system. A further reduction in thesize of the structures on the semiconductor chip would be largelysenseless if there were not a corresponding development in the packagingof the semiconductor chips.

To satisfy this high requirement on the packaging of the semiconductorchips, in recent years packaging methods for semiconductor chips havebeen proposed and developed under the designation “Chip Size Packaging”(CSP), in which the space requirement of the packaged semiconductor chipis not greater than approximately 1.2 times the unpackaged semiconductorchip. Accordingly, the integration density on the system board can beincreased and parasitic effects of the package reduced, wherebyultimately the speed of the end product is significantly increased.

In the production of miniature housings for semiconductor chips,generally a specially preprocessed carrier substrate is used. Thiscarrier substrate may in this case be designed such that it is bothrelatively flexible and relatively rigid_ Generally serving forattaching the semiconductor chip on the carrier substrate are full-areachip bonding tapes or elastomer layers, which in addition to fixing thesemiconductor chip also serve for minimizing mechanical stresses of theoverall arrangement. In particular, in the case of these technologies,the semiconductor chip is permanently connected to the carrier substrateby means of the double-sided adhesive chip bonding tapes or elastomerlayers.

At the same time, packages of this type often have to satisfy extremerequirements with respect to stress absorption. In particular, it isnecessary to compensate largely for thermally induced stresses, causedby different coefficients of expansion (CTE) of individual components ofthe package. In this respect it is generally attempted to reduce thethermomechanical stresses by means of the double-sided adhesive chipbonding tapes or elastomer layers, which to a certain extent havestress-absorbing properties. Unfortunately, so far there is no knownmethod or material that satisfies this requirement satisfactorily.Therefore, instances of delamination in the package or damage to thesemiconductor chip are repeatedly encountered.

SUMMARY OF THE INVENTION

The present invention provides a method of producing a package forsemiconductor chips which largely avoids the difficulties mentioned andwhich makes possible, in particular, a low-cost, virtually stress-freeelectrical bonding of the chip for miniature housings.

anying drawings.

According to one embodiment of the invention, there is a method ofproducing a package for semiconductor chips including a carriersubstrate is provided, at least one adhesive layer, which loses itsadhesive property when it is heated above a defined temperature, isapplied to the carrier substrate, a semiconductor chip is applied to thecarrier substrate and fixed by the adhesive layer, a curable compositionis applied and at least one heat treatment is carried out, thetemperature of the heat treatment lying at least temporarily above thetemperature at which the adhesive layer loses its adhesive property, sothat the adhesive layer loses its adhesive property and the curablecomposition cures.

The invention has one advantage that, during the completion of thepackage, the mechanical connection between and the carrier substrate isreleased again. The mechanical connection between the semiconductor chipand the carrier substrate takes place temporarily. As a result, acritical interface in the package is removed, which has the consequenceof a significant reduction in the thermomechanical stresses.Accordingly, the completed package is significantly more reliable, whichis in turn reflected in a reduction in the costs.

With the method according to the invention, miniature packages orminiature housings for semiconductor chips (CSP) can be produced inparticular.

According to a preferred embodiment of the invention, a curable firstheat treatment is carried out, so that the curable compositionsolidifies, the temperature of the first heat treatment lying above thetemperature at which the adhesive layer loses its adhesive property, sothat the adhesive layer loses its adhesive property and the curablecomposition cures. This preferred embodiment of the invention has anadvantages that, in the second heat treatment, the process parameterscan be set exactly as required for a reliable and permanent release ofthe adhesive layer. At the same time, the second heat treatment actslike a post-curing process for the curable composition.

According to a further preferred embodiment of the invention, before thecurable heat treatment the semiconductor chip is electrically connectedto the carrier substrate by wire or film-carrier bonding (lead bonding,TAB), the bonding temperature lying below the temperature at which theadhesive layer loses its adhesive property. According to a furtherpreferred embodiment of the invention, the adhesive layer is applied bya punching process. Furthermore, it is preferable if a first adhesivelayer, a base layer and a second adhesive layer are applied, at leastthe second adhesive layer property losing its adhesive property when itis heated above a defined temperature. In this case, it is preferred inparticular if the first adhesive layer, the base layer and the secondadhesive layer are jointly applied in the form of a film. According to afurther preferred embodiment of the invention, a polyimide, BT or FR4substrate is used as the carrier substrate. Furthermore, it is preferredif the carrier substrate has at least one bonding channel.

According to a further preferred embodiment of the invention, a mixtureof silicone and epoxy resin is used as the curable composition. The useof a mixture epoxy resin has the advantage over the use of pure epoxyresin that the encapsulating composition has a significantly higherflexibility even after curing. If, however, the production of arelatively rigid package is the aim, it is advantageous to use pureepoxy resin as the curable composition.

According to a further preferred embodiment of the invention, the firstheat treatment is carried out at a temperature between 1300 and 150° C.and subsequently a second heat treatment is carried out at a temperaturebetween 150° and 170° C.

According to a further preferred embodiment of the invention, thenubbins are applied to the carrier substrate by a printing process and asubsequent heat treatment. Furthermore, it is preferred if the curablecomposition is applied by a printing process or a compression-moldingprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is presented in more detail below on the basis of figuresof the drawing, in which:

FIGS. 1-6 show an embodiment of the invention of producing a package forsemiconductor chips.

FIG. 7 shows a plan view of a package produced according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-6 show an embodiment of the method according to the invention ofproducing a package for semiconductor chips. For this purpose, in afirst step a carrier substrate 1, for example a polyimide substrate,which, for later receiving bonding wires, has a central channel 5 (slot)(FIG. 1), is provided. Furthermore, interconnects and landing pads (notshown) for the micro-ball grid array to be produced later are providedon one side of the carrier substrate (underside), whereby the carriersubstrate 1 is connected for example to a system board. In this case,the structure shown in FIG. 1 generally represents only a detail of alarger carrier substrate 1, which serves for the simultaneous attachmentof a number of semiconductor chips. For reasons of overall clarity,however, only the conditions for one semiconductor chip at a time areshown below.

Subsequently, nubbins 3 are applied to the carrier substrate 1, forexample by means of a printing process, and are cured in an ovenprocess. Silicone is used with preference as the material for thenubbins 3. These nubbins 3 also serve for assisting the later wirebonding. Consequently, a carrier substrate 1 with nubbins 3 arranged onit is prepared. The resulting situation is represented in FIG. 2.

Subsequently, an adhesive layer 4 that is released under the influenceof a defined temperature is applied to the nubbins 3. In this respect,in the case of this embodiment of the method according to the invention,the adhesive layer 4 is applied to the nubbins 3 as part of a film by apunching process. A film of this type generally comprises in addition tothe adhesive layer 4 a base layer (not shown), for example polyester,and a further adhesive layer (not shown), the base layer being arrangedbetween the two adhesive layers. Films of this type are available, forexample, under the designation “Revalpha” from the company Nitto Denkc.The situation resulting from this is represented in FIG. 3.

After that, a semiconductor chip 2 is attached on the carrier substrate1 by means of the adhesive layer 4 (FIG. 4) and the electricalconnection between the semiconductor chip 2 and the carrier substrate 1is made in the bonding channel 5 by means of fine connecting wires (notshown), for example gold wires. The bonding temperature in this caselies below what is known as the “heat release” temperature, at which theadhesive layer 4 loses its adhesive property.

The overall construction is then encapsulated in a curable compositionor encapsulating composition 6 adapted to the coefficient of thermalexpansion of the semiconductor chip. For this purpose, the encapsulatingcomposition 6 is applied to the structure shown in FIG. 4 by a printingprocess. A mixture of silicone and epoxy resin is used as theencapsulating composition 6. The use of a mixture of silicone and epoxyresin has the advantage over the use of pure epoxy resin that theencapsulating composition 6 has a significantly higher flexibility evenafter curing. Alternatively, instead of a mixture of silicone and epoxyresin, other materials, for example pure epoxy resin, may also be usedas what is known as compression-molding process.

For the curing of the encapsulating compound 6, after that a first heattreatment is carried out, so that the encapsulating composition 6solidifies, the temperature of the first heat treatment lying below thetemperature at which the adhesive layer 4 loses its adhesive property.The first heat treatment is preferably carried out at a temperaturebetween 130° and 150° C. for approximately one to two hours.

After that, a second heat treatment is carried out, the temperature ofthe second heat treatment lying above the temperature at which theadhesive layer 4 loses its adhesive property, so that the adhesive layer4 loses its adhesive property and the encapsulating composition 6 fullycures. The second heat treatment is preferably carried out at atemperature between 150° and 170° C. for approximately 10 to 15 minutes.This procedure has the advantage that, during the second heat treatment,the process parameters can be set exactly as required for a reliable andpermanent release of the adhesive layer 4. At the same time, the secondheat treatment acts like a post-curing process for the encapsulatingcomposition 6.

After that, a micro-ball grid array 7 (or some other connecting means)is produced on the side of the carrier substrate 1 lying opposite thesemiconductor chip 2, said array serving for the electrical andmechanical bonding of the complete arrangement on a printed circuitboard which is not represented (FIG. 6). FIG. 7 shows a plan view of apackage produced in this way.

The method according to the invention has the advantage that, during thecompletion of the package, the mechanical connection between thesemiconductor chip and the carrier substrate is released again. Theconnection necessary for establishing the contacts between thesemiconductor chip and the carrier substrate takes place onlytemporarily. As a result, a critical interface in the package isremoved, which has the consequence of a significant reduction in thethermomechanical stresses.

1. A method of producing a package for semiconductor chips, comprising:providing a carrier substrate; applying at least one adhesive layer,which loses its adhesive property when it is heated above a definedtemperature, to the carrier substrate; applying a semiconductor chip tothe carrier substrate and fixing it thereto by the adhesive layer; andapplying a curable composition and carrying out at least one heattreatment, the temperature of the heat treatment lying at leasttemporarily above the temperature at which the adhesive layer loses itsadhesive property, so that the adhesive layer loses its adhesiveproperty and the curable composition cures.
 2. The method as claimed inclaim 1, wherein a carrier substrate with nubbins arranged on it isprovided and the adhesive layer is applied to the nubbins.
 3. The methodas claimed in claim 1, wherein a curable composition is applied and afirst heat treatment is carried out, so that the curable compositionsolidifies, the temperature of the first heat treatment lying below thetemperature at which the adhesive layer loses its adhesive property, andsubsequently a second heat treatment is carried out, the temperature ofthe second heat treatment lying above the temperature at which theadhesive layer loses its adhesive property, so that the adhesive layerloses its adhesive property and the curable compositions cures.
 4. Themethod as claimed in claim 1, wherein the semiconductor chip iselectrically connected to the carrier substrate by wire or film-carrierbonding, the bonding temperature lying below the temperature at whichthe adhesive layer loses its adhesive property.
 5. The method as claimedin claim 1, wherein the adhesive layer is applied by a punching process.6. The method as claimed in claim 1, wherein a first adhesive layer, abase layer and a second adhesive layer are applied, at least the secondadhesive layer losing its adhesive property when it is heated above adefined temperature.
 7. The method as claimed in claim 6, wherein thefirst adhesive layer, the base layer and the second adhesive layer arejointly applied in the form of a film.
 8. The method as claimed in claim1, wherein a polyimide, BT or FR4 substrate is used as the carriersubstrate.
 9. The method as claimed in claim 1, wherein the carriersubstrate has at least one bonding channel.
 10. The method as claimed inclaim 1, wherein a mixture of silicone and epoxy resin or pure epoxyresin is used as the curable composition.
 11. The method as claimed inclaim 10, wherein the first heat treatment is carried out at atemperature between 130° and 150° C. and subsequently a second heattreatment is carried out at a temperature between 150° and 170° C. 12.The method as claimed in claim 1, wherein the curable compositions isapplied by a printing process or a compression-molding process.
 13. Themethod as claimed in claim 2, wherein the nubbins are applied to thecarrier substrate by a printing process and a subsequent heat treatment.